Question: $ { {-4} \times \left[\begin{array}{r}{1} \\ {3} \\ {1}\end{array}\right] = {?} } $
Answer: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{r}{-4}\times{1} \\ {-4}\times{3} \\ {-4}\times{1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{r}{-4} \\ {-12} \\ {-4}\end{array}\right]}$